Bump C++ version to C++17

Protobuf versions since 3.6.0 have long had a C++11 dependency; even
more recent versions have picked up an Abseil dependency bumping that
to C++14. Since it is now 2023, defaulting to C++17 is reasonable, so
remove the conditional C++ standards version check that mosh
previously had and replace it with an unconditional C++17 check.

This means that all future commits can use C++17 features. According
to https://en.cppreference.com/w/cpp/compiler_support/17 this means
that minimum effective compiler versions for mosh become:

gcc 7 (May 2017)
clang 5 (September 2017)

This, in turn, implies that future versions of mosh will no longer be
available for RHEL/CentOS 7 (June 2014).

Closes: #1267